Cursing doesn't bother me or a one off complaint, but if they are continually verbally abusive to teammates or opposing players that does it for me. A good game depends on teamwork, and people who are toxic to someone having a bad game make that near impossible.

I also reported one guy who kept buying all our sentry & observer wards as Nature's Prophet and then dropping them all at our opponent's well all game long. The whole time he was pretending to be a helpful teammate. That was a funny troll I admit, but totally cost us the game. And watching his replays I saw he did that for the previous ten consecutive games.